Nitrification is the process by which ammonia is converted to nitrites (NO2-) and then nitrates (NO3-). This process naturally occurs in the environment, where it is carried out by specialized bacteria.
Ammonia is produced by the breakdown of organic sources of nitrogen. Nitrogen is the fourth most abundant element in living things, being a major constituent of proteins and nucleic acids. Municipal wastewaters contain large amounts of organic wastes, so the wastewater will have a high ammonia concentration. With this high concentration of ammonia, the wastewater would harm downstream ecosystems if released. Ammonia is toxic to aquatic life at these concentrations, and the the nitrification process requires oxygen (ammonia contributes to the BOD of the wastewater) so it will use up the oxygen needed by other organisms. It is desirable transform the ammonia as part of the waste treatment process, where aeration and other conditions, such as temperature can be controlled.
The nitrification process is carried out
by two different types of bacteria. Nitrosomonas carry out the first
step of the process, producing nitrite:
These reactions, although thermodynamically
favorable, occur slowly. The rate of these reactions seems to be
limited by the slow growth rates of the bacteria involved. Most nitrifying
bacteria are autotrophic, with carbon dioxide serving as their carbon source.
The growth expressions for these microbes are given as:
These reactions are not very efficient
metabolically, so yields are typically low.
The resulting nitrite is then converted to nitrate by Nitrobacters:
These reactions, although thermodynamically favorable, occur slowly. The rate of these reactions seems to be limited by the slow growth rates of the bacteria involved. Most nitrifying bacteria are autotrophic, with carbon dioxide serving as their carbon source. The growth expressions for these microbes are given as:
These reactions are not very efficient metabolically, so yields are typically low.
Rates of Nitrification
The rates of nitrification reaction are
highly dependent on a number of environmental factors. These include
the substrate and oxygen concentration, temperature, pH, and the presence
of toxic or inhibiting substances. The kinetics can be described
with the Monod expression, and in most cases the rate is limited by the
first step, the oxidation of the ammonia. At steady state, the amount
of NO2- present is usually small and can be ignored
in inital design.
Temperature Dependency: Like all microbes, nitrifying bacteria are temperature sensitive. Rapid changes in temperature do not produce rapid changes in growth rates. A slow adaptation period, with a lower than expected rate follows such changes. See figure for a graph of rate change with rapid changes in temperature. Theoretical models for maximum growth rates of the form usually approximate the observed rates of reaction for temperatures from about 0-20° C. At higher temperatures the growth rate does not increase as the model would suggest, and by 35° the growth rate begins to rapidly fall off toward zero. Click here to see a graph of growth rates with temperature.References
Oxygen Concentration: Nitrifying bacteria are especially sensitive to low oxygen concentrations, and providing enough oxygen in waste water treatment plants is often the most important design consideration.
pH Dependency: pH has strong effect on nitrification rates. The reactions occur fastest when pH is from 8 - 9 (see figure), although if the bacteria exist in flocs or films the pH at the cell surface will be lower than the bulk pH due to the production of H+ ions. The exact mechanisms by which pH effects the reaction rates are not fully understood, although it is believed that inhibitions, particularly from the neutral NH3 and HNO2 species, become important.
Inhibiting Substances: Many substances can potentially inhibit the nitrification reactions. Metals are particularly strong inhibitors of the reactions. When exposed to more than one inhibitor, the extent of inhibition increases greatly.